The present invention relates to ceramic cutting tool inserts and, in particular, to such cutting tool inserts in which monocrystalline whiskers (hair crystals) and/or platelets are homogeneously distributed in an oxide based ceramic matrix, the inserts having preformed chipbreakers for machining of long-chipping materials.
Ceramic cutting materials have now been available for several decades but have not, until recently, had any commercial importance for use in chip-forming machining. The main reason for the limited growth of ceramic cutting tools has been their sudden and unexpected failures because of their inherent inadequate strength and toughness.
During the last decade, the properties of ceramic cutting materials have been improved in many respects resulting in an increased relative share in the cutting of cast iron and nickel-based high-temperature alloys.
During the very recent years, the incorporation of whiskers, or single crystal fibers, has enabled the use of ceramic cutting tools not only in heat resistant alloys but also in the machining of steel, a material which makes extreme simultaneous demands upon strength, toughness, thermal shock and wear resistance.
The latter application, as well as the cutting of heat resistant alloys to some extent, put demands on the chipbreaking capability of the insert. Steel machining generally produces continuous long chips, which unless broken, will create severe problems during the machining operation as they can interfere with all rotating parts and also damage the surface finish of the machined part.
Up till now, this has not been a problem since almost all machining with ceramic tools has been made in workpiece materials producing short interrupted chips, i.e., cast iron. Generally, ceramic inserts also have all surfaces ground flat. In case chipbreaking is necessary, a loose (or separate) chipbreaker normally is attached to the tool holder. A simple chipbreaking geometry may also be ground on the insert.
Now with more tough ceramic materials available, it is not only possible but also necessary to be able to produce more complex shaped insert styles in order to utilize the full potential of the cutting tool materials.
Since the improved properties of more recently developed ceramic materials have been obtained using rather large additions of whiskers, however the processing is not straight-forward. Whisker additions normally mean that simultaneous application of high temperature and pressure is necessary to completely consolidate the material.
Densification of pressed powder-whisker compacts is inhibited because rearrangement of whiskers during shrinkage is difficult. With whisker contents&gt;10 volume %, densification is severely inhibited and sintered materials without closed porosity is obtained, (see T. Tiegs and P. Becher, "Sintered Alumina-Silicon Carbide Whisker Composites", Am. Cer. Soc. Bull., 66(2), 339-342 (1987)).
For metal cutting purposes, whisker contents in the order of 25-35% by volume is necessary to obtain the requested levels of strength, toughness and thermal shock resistance. It is thus necessary to consolidate the material using simultaneously a high temperature and a high pressure.
Uniaxial hot pressing which is the most frequent method for consolidation of difficult to sinter ceramic materials is not applicable in this case since this method can only produce fairly simple geometrical shapes.
Hot isostatic pressing using gas as the pressure medium is further also not applicable since a closed porosity is needed unless the material to be sintered is enclosed in a can. The canning material can be a metal, glass or some other material that can be attached onto the surface of the perform to form a coating which is not penetrable by the gas.